1. Field of Invention
The invention relates to an automatic transmission for a vehicle. More specifically, it relates to a servo pressure application to each clutch in the gear train.
2. Description of Related Art
An automatic transmission, for which a power source is, for example, an engine and which used for driving, is required to multiply the gear ratios for an improvement in energy conservation. The energy conservation is improved by an efficient power transmission corresponding to a load. According to the requirement to conserve energy, the transmission mechanism of the automatic transmission for a vehicle has gone from a mechanism having forward four gear stages to a mechanism having forward five gear stages. For achieving a further increase in the gear stages in the limited mounting space on a vehicle, a reduction in the number of elements of the gear train and a simplification of the mechanism are needed. A gear train, which achieves six forward speeds and one reverse speed with a planetary gear set having minimum shift elements, three clutches and two brakes operating the planetary gear set, is suggested in U.S. Pat. No. 5,106,352. In this gear train, six forward speeds are established by two kinds of input rotations having different speeds, which are an engine output rotation, that is, a turbine output rotation of a torque converter and a decelerated rotation decelerated from that, are properly inputted to the planetary gear set having four shift elements of the shift mechanism by using three clutches and two shift elements are engaged with a transmission case by two brakes.
In the aforementioned gear train, the number of shift elements for establishing gear stages and the number of required clutches and brakes are reasonable. Generally, seal rings are arranged in a hydraulic path, for applying a hydraulic pressure to the clutches and the brakes, for sealing at a position at which members relatively rotate. The seal ring provides a seal effect by being pressed. Therefore, relatively large sliding resistance occurs between the members which relatively rotate with respect to each other. Therefore, if the number of seal rings increases, the rotational resistance of corresponding rotational members increases and power loss increases. Further, grooves, in which seal rings are arranged, need high accuracy because the oil is sealed by contacting seal rings and grooves. Additionally, wear can occur in portions relatively sliding with seal rings by the relative rotation to the seal rings. Therefore, such portions need improvement, such as a hardening to prevent wear. Therefore, if the number of seal ring grooves increases, man hours for manufacture and manufacturing cost increase producing a product cost increase. Thus, it is desirable to decrease the number of seal portions.
In the aforementioned gear train in the related art, there are two input side power transmission routes in which an output from the torque converter is inputted as it is to the planetary gear set in one side, the output from the torque converter is decelerated at a reduction planetary gear and the decelerated rotation is inputted to the planetary gear set. Therefore, there may be a need for many seal rings corresponding to an arrangement of the hydraulic servo of each clutch, which changes the power transmission route for shifting, because the connecting arrangement for the hydraulic paths, for applying hydraulic pressures, from the transmission case to the hydraulic servos is complicated and the hydraulic paths traverse many times between members relatively rotated with respect to each other.
Generally, a hydraulic pressure is applied to the hydraulic servo of the each clutch from a front end wall portion and a rear end wall portion of the transmission case. The hydraulic path structure is simplified by arranging a center support on the middle portion of the transmission case and applying hydraulic pressure from the center support. In this case, the number of seal rings is reduced. However, in such a case, the center support is arranged in the shift mechanism. Therefore, the axial size of the transmission is increased. Further, the number of parts is increased and costs increase. Therefore, it is desirable to simplify the hydraulic path structure. Especially, the aforementioned gear train in the related art would have a multiple shaft structure because there are two kinds of inputs having different rotational speeds. Therefore, it is difficult to greatly reduce the number of seal rings even if the center support is provided.